In screw-spindle compressors, as disclosed by EP-A 472933, the pressure difference attainable depends to a considerable extent on the leakage losses between the peripheral surfaces, moving relative to one another, of the rotors and the pump-chamber housing. In view of this, the aim will be to keep the clearance between these surfaces as small as possible. However, the operating safety, with due regard to the temperature-induced thermal expansion of the rotors, requires greater clearance.
It is known to directly cool rotors of twin-shaft compressors (EP-A 290664) by a heat-transfer medium (lubricating oil) being provided in a bearing hollow space of the rotor, which heat-transfer medium is cooled by a stationary cooling coil projecting into the bearing hollow space. This has the disadvantage that the bearing hollow space of the rotor has to be sealed off. However, the seals required for this are trouble-prone, in particular at a high number of revolutions. High losses which lead to the generation of heat and jeopardize the cooling effect also arise in the heat-transfer medium, which is swirled between the rotating rotor and the stationary cooling coil.
It is conventional practice to cool the delivered medium by liquid coolant, for example, being injected (U.S. Pat. No. 4,515,540) or by some of the delivered medium being fed back after cooling (DE-A 25 44 082). Such cooling may also be provided in combination with the invention; however, the aim of the invention is to cool the rotor so that the rotor, in particular in the area of the sensitive bearings, can assume a temperature which is below the pressure-side temperature of the delivered medium.